Hydroformylation reactions involve the preparation of oxygenated organic compounds by the reaction of carbon monoxide and hydrogen (synthesis gas) with carbon compounds containing olefinic unsaturation. The reaction is typically performed in the presence of a carbonylation catalyst and results in the formation of compounds, for example, aldehydes, which have one or more carbon atoms in their molecular structure than the starting olefinic feedstock. By way of example, higher alcohols may be produced by hydroformylation of commercial C.sub.6 -C.sub.12 olefin fractions to an aldehyde-containing oxonation product, which on hydrogenation yields the corresponding C.sub.7 -C.sub.13 saturated alcohols. The crude product of the hydroformylation reaction will contain catalyst, aldehydes, alcohols, unreacted olefin feed, synthesis gas and by-products.
A variety of transition metals catalyze the hydroformylation reaction, but only cobalt and rhodium carbonyl complexes are used in commercial oxo plants. The reaction is highly exothermic; the heat release is ca 125 kJ/mol (30 kcal/mol) The position of the formyl group in the aldehyde product depends upon the olefin type, the catalyst, the solvent, and the reaction conditions. Reaction conditions have some effect and, with an unmodified cobalt catalyst, the yield of straight chain product from a linear olefin is favored by higher carbon monoxide (CO) partial pressure. In the hydroformylation of terminal olefinic hydrocarbons, the use of a catalyst containing selected complexing ligands, e.g., tertiary phosphines, results in the predominant formation of the normal isomer.
In commercial operation, the aldehyde product used as an intermediate is converted by hydrogenation to an alcohol or by aldolization and hydrogenation to a higher alcohol. The aldol-hydrogenation route is used primarily for the manufacture of 2-ethylhexanol from propylene via n-butyraldehyde.
The hydroformylation reaction is catalyzed homogeneously by carbonyls of Group VIII metals but there are significant differences in their relative activities. Roelen, using a cobalt catalyst, discovered hydroformylation in 1938. Dicobalt octacarbonyl, Co.sub.2 (CO).sub.8, which is introduced either directly or formed in situ, is the primary conventional hydroformylation catalyst precursor. Using an unmodified cobalt catalyst, the ratio of linear to branched aldehyde is relatively low.
Introduction of an organophosphine ligand to form a complex, e.g., Co.sub.2 (CO).sub.6 [P(n--C.sub.4 H.sub.9).sub.3 ].sub.2, significantly improves the selectivity to the straight-chain alcohol.
Recent developments of low pressure rhodium catalyst systems have been the subject of a considerable body of patent art and literature, and rhodium-triphenyl phosphine systems have been widely, and successfully, used commercially for the hydroformylation of propylene feedstocks to produce butyraldehyde.
The first commercial process to employ a rhodium-modified catalyst was developed by Union Carbide, Davy Powergas, and Johnson Matthey. In this application, the complexed rhodium catalyst is dissolved in excess ligand and the reaction is run at relatively low pressures and temperatures as compared to other processes.
A recent process commercialization has been that of Rhone-Poulenc and Ruhrchemie which produces butyraldehyde from propylene but the ligand is a sulfonated triphenylphosphine and is utilized as a water-soluble sodium salt. Turnover rates are less than in an all-organic system, but the normal to iso ratios are high and the catalyst may be separated easily from the reaction product by separation of the aqueous layer containing the catalyst and the organic layer which constitutes the product.
In the formation of linear aldehydes using a ligand-modified rhodium-catalyzed homogenous process, the reactor comprises the rhodium complex catalyst, excess triphenylphosphine and a mixture of product aldehydes and condensation by-products. The product aldehyde may be recovered from the mixture by volatilization directly from the reactor or by distillation in a subsequent step. The catalyst either remains in or is recycled to the reactor. However, the complex catalyst and triphenylphosphine ligand are slowly deactivated and eventually the spent catalyst is removed for recovery of rhodium and reconversion to the active catalyst. This process, although effective for lower molecular weight aldehyde production, is not favored for higher molecular weight aldehydes. Higher molecular weight aldehydes have higher boiling points (i.e., distillation temperatures) and catalyst deactivation is accelerated.
In some cases, such as where the products of the reaction have relatively high boiling points or where the olefin feed is not sufficiently soluble in water to permit satisfactory reaction rates, neither the process where the products are removed from the catalyst by distillation or stripping nor where the products are decanted from an aqueous catalyst solution may be utilized successfully. In such cases, it may be advantageous to utilize an aqueous medium to contain the catalyst and add a surfactant to enhance phase contacting so as to improve rate and selectivity to the desired products. This type of process is called "Phase Transfer Catalysis." However, when a surfactant is added, some carry-over of the noble metal into the organic phase at the conclusion of the process often results.
The present inventors have discovered that when olefins are satisfactorily hydroformylated in the presence of water-soluble Group VIII noble metal-ligand complex catalysts, the catalyst can be recovered quantitatively from a crude reaction product which includes the olefinic feed, aldehydes and alcohols by employing membrane separation either internal or external to the hydroformylation reactor.
A variety of membrane separation processes have also been tested for separating high boiling point products from an oil soluble catalyst complex. Attempts have been made to create large catalyst complexes which could be separated by ultrafiltration. In one case, high molecular weight phosphine ligands were used to form a homogeneous catalyst complex. High molecular weight polymeric phosphine ligands are synthesized by reacting polyvinylchloride, polychloroprene or brominated polystyrene with lithium diphenylphosphide at 20.degree. C to 25.degree. C. These homogeneous catalysts containing bulky ligands are thought to be more easily separated from the reaction products by ultrafiltration. See Imyanitov et al., All-Union Scientific Research Institute of Petrochemical Processes, Neftekhimiva, 32, No. 3:200-7 (May-June 1992). The process described herein uses a smaller catalyst complex which is not attached to a polymeric backbone.
It has been known to use membranes to separate catalysts from an aqueous solution. An example is set forth in European Patent No. 0 263 953, published on Aug. 29, 1986 (assigned to Ruhrchemie Aktiengesellschaft), which discloses a process for separating rhodium complex compounds, which contain water-soluble organic phosphines as ligands, from aqueous solutions in which excess phosphine ligand and, if necessary, other components are also dissolved, is characterized by the fact that the aqueous solution is subjected to a membrane separation process. According to this process, volatile organic substances are separated from the solution prior to conducting the membrane separation process. A typical membrane for use in this process is a cellulose acetate membrane. This process only involves the separation of water-soluble ligands and noble metal catalyst from an aqueous solution. As such, this separation process does not pertain to the separation of a water-soluble noble metal catalyst and a water-soluble ligand from an organic-aqueous emulsion, dispersion or suspension produced from a hydroformylation process.
Another patent which utilizes cellulose acetate, silicone rubber, polyolefin or polyamide membranes in the separation of catalysts from high boiling by-products of the hydroformylation reaction is Great Britain Patent No. 1312076, granted on May 15, 1970. According to this patent the aldehydes produced during the hydroformylation process are continuously withdrawn as an overhead vapor stream. The liquid stream containing the heavy by-products with the catalyst is passed over a membrane wherein approximately 78-94.3% of the catalyst is retained and the heavy by-products permeated. This is an unacceptably low level of catalyst retention which is overcome by the process of the present invention.
In like manner, Great Britain Patent No. 1432561, granted on Mar. 27, 1972, (assigned to Imperial Chemical Industries Ltd.) discloses a process for the hydroformylation of olefins which comprises reacting an olefin at elevated temperature and pressure with CO and H.sub.2 in the presence of a compound of a group VIII metal and a biphyllic ligand of a trivalent P, As or Sb to give a crude liquid hydroformylation product containing an aldehyde and/or an alcohol, separating the aldehyde and/or alcohol from the crude product and leaving a liquid, bringing the liquid after separation of the Group VIII metal compound and free from aldehyde and alcohol under reverse osmosis conditions into contact with one side of a silicone rubber semi-permeable membrane in which the polymer chains have been at least partly crosslinked by gamma radiation, whereby the liquid retained by the membrane contains a higher concentration of Group VIII metal compounds and/or biphyllic ligand than the original liquid.
In an article by Gosser et al., entitled "Reverse Osmosis in Homogeneous Catalysis," Journal of Molecular Catalysis, Vol. 2 (1977), pp. 253-263, a selectively permeable polyimide membrane was used to separate soluble transition metal complexes from reaction mixtures by reverse osmosis. For example, separation of cobalt and rhodium complexes from hydroformylation products of 1-pentene. That is, a solution of 0.50 grams of RhH(CO)(PPh.sub.3).sub.3 in 40 ml of benzene and 10 ml of 1-pentene was stirred at 50.degree. C. with a CO/H.sub.2 mixture at ca. 4 atm pressure until no further pressure drop occurred. The pentene was completely converted to aldehydes according to proton nuclear magnetic resonance (nmr) analysis. The solution was permeated through a polyimide membrane under 68 atm nitrogen pressure. The permeate (4.5 grams passed in 2 min.) showed only 9% of the original rhodium concentration by X-ray fluorescence.
The permeation rate of rhodium as set forth above, i.e., 9%, is considered unacceptable. The rhodium catalyst should be retained in an amount of greater than 99% to be a commercially feasible process.
Another example of the use of membranes to separate metal catalysts from hydroformylation products is set forth in Dutch Patent No. 8700881, published on Nov. 1, 1988. The method disclosed therein relates to one which improves the efficiency of membrane separation of hydroformylation products from expensive organometallic catalyst containing reaction mixtures. In Dutch Patent No. 8700881 a polydimethylsiloxane membrane having a thickness of 7 microns applied to a Teflon.RTM. support was used in the separation of a reaction mixture containing C.sub.9-C.sub.15 alcohols, a homogeneous catalyst system comprising an organometallic complex of a transition metal from Group VIII or VIIa or Va of the Periodic Table, e.g., a tricarbonyl (triphenyl-phosphine) cobalt catalyst, and 40% low-viscosity lubricating oil (an antiswelling or de-swelling agent). At a flow of 133 kg/m.sup.2 -day, the cobalt contents in the feed, retentate, and permeate were 600, 910, and 18 ppm, verses 840, 1930, and 160 ppm, respectively, for a mixture without the de-swelling agent. This process is directed to the separation of product from a reaction mixture containing a homogeneous catalyst system by means of a membrane. The ligands disclosed in Dutch Patent No. 8700881 are all organic soluble ligands, e.g., triphenylphosphine, tri-n-alkylphosphine or acetyl acetonate. Critical to the process of Dutch Patent No. 8700881 is the addition of a de-swelling agent to the reaction mixture which assists in the separation of the products from the reaction mixture.
Each of the aforementioned processes for removing metal catalysts from crude hydroformylation reaction products are both costly in terms of unrecovered catalyst and, as such, would require further expensive treatment of the streams to recover catalyst.
The present inventors have been examining whether rhodium separation from hydroformylation products can be performed with a membrane when the catalyst complexes are formed using hydrocarbon or oil soluble phosphine ligands in the presence of an atmospheric mixture of CO and H.sub.2 and also whether such a separation can be effected using water-soluble phosphine ligands at higher than atmospheric pressures of CO and H.sub.2. For the organic-soluble system, they have discovered that alkylated phosphine ligands together with dense nonpolar polymeric membranes are capable of substantially retarding the rhodium loss during the separation of the rhodium catalyst from the hydroformylation reaction products. It was also discovered that triphenylphosphine ligands used in conjunction with a dense polymeric, nonpolar membrane also substantially retards rhodium catalyst loss, although not as well as alkylated phosphines. Optimum operating conditions for the organic-soluble system involve performing the separations in an atmosphere of CO and H.sub.2 each with partial pressures less than one atmosphere.
For the water-soluble system the present inventors have discovered that rhodium catalysts may be separated from the reaction products using a variety of ligands and a variety of hydrophobic membranes.
In processes which produce high molecular weight products, it may not be possible to provide the driving force needed to drive the products through the membrane by pervaporation due to the need for high temperatures and high vacuum and thus perstraction would be desirable.
Perstraction typically involves the selective dissolution of particular components contained in a mixture into the membrane, the diffusion of those components through the membrane and the removal of the diffused components from the downstream side of the membrane by use of a liquid sweep stream.
For example, in perstractive separations of aromatics from saturates in petroleum or chemical streams (particularly heavy cat naphtha streams) the aromatic molecules present in the feedstream dissolve into the membrane film due to similarities between the membrane solubility parameter and those of the aromatic species in the feed. The aromatics then permeate (diffuse) through the membrane and are swept away by a sweep liquid which is low in aromatic content. This keeps the concentration of aromatics at the permeate side of the membrane film low and maintains the concentration gradient which is responsible for the permeation of the aromatics through the membrane.
The sweep liquid should be low in aromatic content so as not to itself decrease the concentration gradient. The sweep liquid is preferably a saturated hydrocarbon liquid with a boiling point much lower or much higher than that of the permeated aromatics. This is to facilitate separation, as by simple distillation. Suitable sweep liquids are C.sub.3 to C.sub.6 saturated hydrocarbons and lube basestocks (C.sub.15 -C.sub.20).
The perstraction process is run at any convenient temperature, preferably as low as possible.
The choice of pressure is not critical since the perstraction process is not dependent on pressure, but on the ability of the aromatic components in the feed to dissolve into and migrate through the membrane under a concentration driving force. Consequently, any convenient pressure may be employed, the lower the better to avoid undesirable compaction, if the membrane is supported on a porous backing, or rupture of the membrane, if it is not.
If C.sub.3 or C.sub.4 sweep liquids are used at 25.degree. C. or above in liquid state, the pressure must be increased to keep them in the liquid phase.
A disadvantage of conventional perstraction methods, however, is the need to provide a step to remove the perstraction sweep stream that has permeated (diffused) to the reaction side from the permeate side of the membrane.
The present inventors have developed a method which overcomes the problems associated with diffusion of the sweep stream and contamination of the reactants and products located on the reactor side of the membrane. This method uses the hydroformylation feedstock, e.g., olefins, as the sweep stream. Thus, it is also possible that the hydroformylation feed may be supplied to the reactor, in addition to the conventional feed method, by diffusion of the sweep stream through the membrane such that it is delivered to the hydroformylation reactor together with the retentate recycled to the reactor after contacting the membrane separator.
The present invention also provides many additional advantages which shall become apparent as described below.